Relaxin, a polypeptide product of the human corpus luteum of pregnancy, is present in maternal serum throughout pregnancy. An understanding of the control of relaxin secretion will provide useful information of the factors that modify and effect corpus luteum activity. We have shown that hCG can stimulate relaxin secretion in nonpregnant luteal phase women. These in vivo studies will be continued to determine the effects of steriods and other hormones on the maintenance of relaxin secretion. We have developed an in vitro system of long term human luteal cell maintenance. This system will be used to determine factors necessary for induction of relaxin secretion from corpora lutea of nonpregnant women and maintenance of relaxin secretion by corpora lutea of pregnancy. Human relaxin will be purified using HPLC. This purified relaxin will be used to determine the interrelationships of human relaxin and other agents on human myometrium in vitro. Progesterone and relaxin synergize in inhibition of myometrial contractin amplitude. A sensitive human bioassay for relaxin will be developed utilizing myometrium pretreated with subeffective doses of progesterone. Decidua may be an additional source of human relaxin. We will quartitate and characterize decidual relaxin and study its control. This material may provide relaxin for human RIA production.